Various devices are employed for detecting force or change in force and other characteristics related thereto. Exemplary of the range of such devices are force platforms in the biomedical, sports and industrial fields measuring large scale ground reaction forces, and seismographic mattresses in the medical fields detecting small scale cardiovascular and respiratory forces.
Force platforms, in particular, measure ground reaction forces in biomedical applications such as gait analysis, analysis of sway patterns in neurology, evaluation of sports performance, assessment of the degree of rehabilitation and numerous other medical, sports and industrial applications. These platforms typically use multiple strain gauges, capacitive sensors or load cells for sensing force and changes in force. Due in part to the inherent size limitations of the sensors, they cannot directly act as the load receiving surface and must be used in arrays and coupled to a platform surface, generally a rigid, light and very stiff planar member. One sensor for such force platforms is disclosed in U.S. Pat. No. 3,210,993 wherein a ceramic piezoelectric transducer encircles a solid metal cylindrical support column. The transducer is stressed in accordance with the radial expansion of the column caused by applied loading to the platform to generate a signal in accordance therewith. To achieve uniformity across the platform surface, complex circuitry is required to interpret the multiple sensor signals generated at each column.
For applications such as seismographic mattresses, rigid supporting surfaces are, of course, not practical. More resilient, compliant materials are required for patient comfort and to provide an accommodating surface which allows for intimate coupling between the patient and the transducer device. Seismographic mattresses are useful for monitoring patients' heart and respiration rates. These mattresses are non-invasive transducers for measuring these biological signals, thereby eliminating the need for restricting and psychologically intimidating sensor attachment to the patient. Typically a sheet of transducer material is placed under the mattress to measure these minute biological forces. The sensor material may be piezoelectric or capacitive film or the like. Various systems using such materials have been proposed for monitoring patient movement and functions such as respiratory and cardiovascular activity. One typical approach to patient monitoring is disclosed in U.S. Pat. No. 4,320,766 wherein a medical apparatus for the monitoring of patient activity on a bed comprises a flexible capacitive motion sensor placed under the patient mattress. The sensor uses an active layer, underlying the entire bottom surface of the mattress. The sensor consists of two courses of dielectric material placed in contact with one another. Movements of the patient on top of the mattress cause the courses to move relative to one another, thereby generating an electric charge in the active layer. The electrical charge is detected by a pair of metallized plastic sheets which form a capacitive antenna. The resultant system is a sandwich of five discrete, mattress size layers all of which are shielded in a metallized plastic film bag. Another patient monitoring device is disclosed in U.S. Pat. No. 4,438,771 wherein a pair of conductive layers connected to a monitoring circuit are spaced by an insulating layer, the layers being substantially coextensive with the bottom surface of the patient mattress. A further approach is disclosed in U.S. Pat. No. 4,509,527 wherein a transducer sheet underlying the patient mattress comprises a piezoelectric sheet engaged on either side between a pair of plastic sheets having interengaging convex surfaces. Patient movement flexes the piezoelectric sheet at the convex surfaces to generate an output signal proportional to the magnitude of the changing mechanical forces which induce the electrical charges. Each of the foregoing patient monitoring devices requires a plurality of operative layers of generally expensive material, typically as much as 20 square feet for adult sized mattresses. Further, the monitoring sensor is normally located under the mattress resulting in significant attenuation of the already minute biological signals to be detected. Also, the mounting of the sensor under constantly flexing loading conditions can accelerate aging wear of the sensor materials due to abrasion.
In view of the foregoing limitations of force measuring devices, it would be desirable to provide a force platform which produces uniformity and repeatability of force measurement, and changes therein, over the entire supporting surface. In other words, each time an identical event occurs, regardless of the point of loading, the output information should be the same. Further, it would be desirable to provide a force platform operable over a wide range of forces. Moreover, such devices should be rugged and durable, able to withstand the forces encountered in operation without damage to the sensors. Furthermore, the the amount of sensor material required to measure large sensing areas should be reduced.